Generator load and starting kVA calculator

Field use cases

Generator discussions often begin with a load list that is not yet ready for product selection. A contractor may have a backup board schedule with lighting, control circuits, a compressor and one pump that must start after the generator is already carrying other load. An engineer may have a temporary-supply request where a hoist or pump start is the event most likely to drive the conversation with the generator supplier. An estimator may need a transparent record that separates running load from starting load before costs or equipment details are discussed.

This calculator is built for that early record. It does not choose the generator. It does not decide whether the alternator can hold voltage during the start. It does not decide engine response, protection settings, changeover equipment, earthing, neutral switching, fuel system, enclosure or site authority matters. Its job is narrower: record the load already running, record one motor-starting event, calculate the apparent-power demand during that event and keep the assumptions visible.

The useful output is the record as a whole. The starting-event kVA by itself is easy to copy into a note, but the value only means something when the connected-load assumption and starting multiplier travel with it.

Load sequence

The calculator assumes one selected motor starts while the other entered load remains connected. That is a common worksheet pattern because generator starting discussions are usually driven by sequence: what is already running, what starts next, and whether any load can be delayed or shed before the motor starts.

If the site has several possible sequences, run the record more than once. A pump starting after refrigeration and lighting are connected can produce a different result from the same pump starting before those loads. A temporary board may have a practical operating sequence that differs from a worst-case list. That sequence should be recorded before the result is used.

Starting assumptions

The multiplier is the most sensitive input. A DOL assumption may be appropriate for an early conservative note, but it should not survive unchanged when motor, starter, drive or generator manufacturer data is available. Reduced-voltage starters, soft starters and drives can reduce the entered current assumption, but the actual outcome depends on settings, load torque, ramp, bypass arrangement and equipment limits.

The multiplier should be visible in the exported record because it is often the first value challenged in a generator review. A supplier can work with a clear assumption. A hidden assumption creates a weak record.

Reading the result

The primary result is starting-event kVA. It combines the running load already on the generator with the selected motor starting kVA. The secondary values show whether the result is driven mainly by other running load, motor starting current or the entered comparison value.

When the starting event is above the entered comparison value, the useful response is not to label the generator as rejected by the website. The useful response is to check whether the running-load list is correct, whether the motor sequence can change, whether the multiplier is too conservative or too optimistic, and what the supplier says about alternator and engine performance.

When the starting event is within the entered comparison value, the record is still only arithmetic. Generator sets can have different short-duration motor-start capability, voltage regulation, engine response, overload behaviour and control settings. A kVA comparison can organise the discussion, but it cannot replace product data.

Practical workflow

Begin with the load list. Remove loads that will not run during the selected motor start, but keep loads that remain connected in the proposed sequence. Then identify the motor start that drives the discussion. For many sites, this is the largest motor. For some sites, it is the first-starting motor because the connected-load condition is different at that moment.

This workflow is useful for backup supplies, maintenance planning, temporary boards and early tender notes. It is less useful where a detailed generator study already exists. In that case, the manufacturer or engineer may already have alternator transient data, step-load data, load bank requirements, protection settings and sequence controls. Those documents should control the project record.

Worked field example

A workshop backup-supply record has 35 kVA of other load running before a fire pump starts. The pump is three-phase at 400 V and has a full-load current of 32 A. The starting method is entered as DOL with a 6 x multiplier. The comparison value entered by the user is 150 kVA.

The starting event is above the entered comparison value. That does not select or reject a generator set by itself. It tells the project team where the pressure is: DOL starting assumption, pump sequence, other connected load, supplier motor-start capability and project voltage behaviour. If the pump can start before other load is connected, the other running kVA changes. If soft-start data is available, the multiplier changes. If the generator supplier confirms a short-duration capability for the specific set, the comparison context changes.

Boundary with related calculators

The generator worksheet sits between motor calculations and wider load review. It should not absorb those jobs.

Keeping those boundaries clean helps the generator page stay useful. A generator supplier does not need a page that pretends to know product curves. They need a clear record of loads, sequence and assumptions.

Stop points

• The entered running load is only a rough site total and does not match the proposed generator sequence.
• Several motors may start together, but the worksheet includes only one selected motor.
• The multiplier is a preset even though product data is available.
• The comparison kVA is copied from a product nameplate without checking motor-start capability.
• The load includes equipment with sensitive voltage, frequency or harmonic requirements.
• The project needs changeover, earthing, MEN, neutral switching, protection or RCD decisions.
• The generator may operate in parallel with another supply or export arrangement.
• Local authority, DNSP, manufacturer or engineering instructions provide a different requirement.

When one of those stop points appears, the calculator has reached its limit. Keep the arithmetic record, then move the decision to the current standards, authority requirements, manufacturer data and project reviewer responsible for the installation.